Radiant energy imaging with rocking scanning

ABSTRACT

A pencil beam of X-rays scans an object along a line of direction along a line X-ray detector to produce an image of the line along a picture tube. The object scanned and the line of scan are relatively displaced by rocking the X-ray source and line detector about an axis substantially passing through a slit which collimates the X-ray energy into a fan beam intercepted by a rotating disc with radial slits that provides the scanning pencil beam. The X-ray detector preferably comprises sodium iodide elements imbedded in a light pipe with a photocell at each end of the light pipe.

o E United States atent 1 [111 eme Stein et al. 45 F b, 5, 1974 [5RADlAN'l ENERGY IMAGING WITH 3,275,831 9/1966 Martin 250/105 arcla er[75] Inventors: Jay A. Stein, Framingham; Gerald I Poluccl, Cambridge,both of Mass. P imary Examiner Archie R Borchelt I V g [73] Assignee:American Science & Engineering Attorney, Agent, or Firm-Charles Hieken &Jerry lnc., Cambridge, Mass. Cohen [22] Filed: June 21, 1972 21 App].No.: 264,857 [57] ABSTRACT A pencil beam of X-rays scans an object alonga line of direction along a line X-ray detector to produce an '5 250/363665 2 image of the line along a picture tube. The object [58] Fieid 5 S91 scanned and the line of scan are relatively displaced 5 6 1 byrocking the X-ray source and line detector about an axis substantiallypassing through a slit which collimates the X-ray energy into a fan beamintercepted by [56] References Cited a rotating disc with radial slitsthat provides the scan- UNITED STATES PATENTS ning pencil beam. TheX-ray detector preferably com- 2,491,224 12/ 1949 Stava 250/616 prisessodium iodide elements imbedded in a light pipe 21 igzag g with aphotocell at each end of the light pipe. 0 en 10 Claims, 4 DrawingFigures MODULATION 0|sc IMAGE STORAGE AND DISPLAY SLIT COLLIMATORRADIANT ENERGY IMAGING WITH ROCKING SCANNING BACKGROUND OF THE INVENTIONThe present invention relates in general to radiant energy imaging andmore particularly concerns novel apparatus and techniques for displayinga visual image of concealed objects on a person with sufficientresolution to identify the object while keeping the intensity ofradiation relatively low. The system is reliable, relatively economicaland may be operated by relatively unskilled personnel.

The problem of detecting contraband concealed in packages and on personsis a serious one. X-ray equipment is useful for assisting in thediscovery of concealed contraband. Conventional X-ray equipment iscostly, requires operation by skilled personnel and may well subjectpersonnel and parcels to undesired excessive dangerous radiation.

American Science & Engineering, Inc. has supplied a parcel inspectionsystem to the US. Postal Laboratory at Rockville Maryland. That systemincludes means for scanning a line X-ray detector with a pencil beam ofX-ray energy to provide a line image signal characteristic of the X-rayresponse of the parcel being scanned, and a television display systemfor displaying the sequenc of image signals derived on the line detectoras the parcel moves past. The detector comprises a sodium iodide crystalthat produces a visible manifestation of the intensity of the incidentradiation that is sensed by a photodetector to provide a characteristicvideo output signal that may be applied to the television display havingan image storage tube. American Science & Engineering, Inc., had alsoproposed using the invention for scanning people by moving the X-raysource and detector together vertically and thereby scan a person.

It is an important object of this invention to provide apparatus andtechniques that retain the advantages of the invention discussed abovewhile providing additional advantages.

It is a further object of the invention to provide an improved personnelinspection system in accordance with the preceding object.

It is still a further object of the invention to achieve one or more ofthe preceding objects with an improved detector.

-It is still a further object of the invention to achieve one or more ofthe preceding objects with apparatus that is relatively inexpensive andeasy to fabricate and use while operating reliably with relativelyunskilled personnel.

SUMMARY OF THE INVENTION According to the invention, there is means forscanning a radiation sensitive detector along a curve with a pencil beamof radiation to provide a line image signal characteristic of radiantenergy response between the source of the pencil beam and the radiationsensitive detector, means for displaying the image represented by theimage signal, slit means comprising the source of the pencil beam forproviding a fan beam, 21 source of radiant energy. and means forangularly displacing thesource of radiant energy and the radiationsensitive detector together about an axis included by the slit meanswhile maintaining the detector and radiant energy source in fixedrelationship.

Preferably the detector comprises at least one segment of a sodiumiodide or cesium iodide crystal imbedded in a light pipe that produces avisible manifestation of the intensity of the incident X-ray radiation,the light pipe preferably having a photocell at each end to collectvirtually all the light energy provided by the crystals in response toradiant energy incident thereon.

Numerous other features, objects and advantages of the invention willbecome apparent from the following specification when read in connectionwith the accompanying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a pictorial representation ofa person inspection system according to the invention;

FIG. 2 is a sectional view through section 22 of FIG. 1 illustrating thepreferred form of the slit cross section along the axis of the scanningbeam;

FIG. 3 is an alternate slit section; and

FIG. 4 is a perspective view of a preferred form of detector.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference now to thedrawing and more particularly FIG. 1 thereof, there is shown a pictorialrepresentation of a preferred embodiment of the invention that isespecially convenient for personnel inspection. A person 11 to beinspected stands on a platform 12 of material that is transparent toX-rays, and the assembly scans the sector 13 to provide an X-ray image14 on image and storage display device 15 that reveals the presence ofcontraband on the person 11 or in the parcel 16 that he is carrying.

Line detector 21 is carried by long arm 22 which support X-ray tube 23at the other end, the assembly being pivotal about an axis substantiallypassing through the slit of slit collimator 24. As the sector 13 isscanned, detector 21 assumes a lowermost position 21' with arm 22 inposition 22 and X-ray tube 23 in position 23. The other extreme ofsector 13 is with detector 21 in position 21", arm 22 in position 22"and X-ray tube 23 in position 23".

A modulation disc 25 is positioned adjacent to a slit collimator 24 androtates to produce pencil beam 26 that scans line detector 21 from leftto right as disc 25 rotates in a clockwise direction. Slit collimator 24and modulation disc 25 are made of X-ray opaque material and formed withslits that may be covered with X-ray transparent material. The radialslits, such as 27, are deep enough to provide an X-ray transparentopening opposite the slit in the slit collimator as each radial slit 27moves across the horizontal slit in collimator 24.

Mechanical details of the means for supporting the various elements arenot illustrated so as not to obscure the principles of the invention.The mechanism for effecting the angular scan across the sector 13 arewell known in the art and not a part of the invention. Similarly, thetechniques for rotating and supporting modulation disc 27 are well knownin the art and not a part of the invention.

The geometry and timing of the system is arranged so that each radialslit causes a new pencil beam to strike the left of detector 21 justafter the previous pencil beam has swept pass the right end of thedetector. That is to say, the width of the fan beam provided by slitcollimator 24 corresponds substantially to the separation betweenadjacent ones of the radial slits at substantially the maximum radialdistance from the edge of the disc where the slits intercept the fanbeam. While FIG. 1 shows the elements that provide the scanning pencilbeam in a form to better illustrate the principles of the invention, theelements 23, 24 and 25 are preferably housed relatively close togetherin an enclosure that shields radiation so that the only significantradiation energy that escapes is that in pencil beam 26.

The person being scanned differentially attenuates the X-rays in pencilbeam 26 incident upon detector 25 so that the electrical signal providedon output line 28 is amplitude modulated in proportion to theinstantaneous X-ray flux incident upon it. This signal thus correspondsto a horizontal line image of the transmissivity ofperson l1 and parcel16 and is analogous to one scan line of a television video signal. Asthe assembly moves angularly through sector 13, sequential pencil beamsintercept slightly displaced regions of person 11 and parcel 16 so thatthe corresponding electrical signals from detector 21 may beappropriately displayed lineby-line to produce a two-dimensional imageof the person 11 and the parcel 16 being carried in X-rays analo gous tothe display of a picture on a television monitor as formed byline-by-line images. The output of detector 21 is thus a video signalthat may be processed in accordance with the same storage and displaytechniques used in conventional video systems to store and displaysingle raster images. Since these techniques are well known in the art,further discussion of them is unnecessary here.

Although detector 21 is shown behind the person being scanned forresponding to the radiant energy transmitted through the person, it iswithin the principles of the invention to position the detector in theregion between the radiant energy source and the person being scanned torespond to the scattered energy. This arrangement helps the apparatusdetect concealed objects having different scattering characteristicsfrom their surroundings. Moreover, a system according to the inventionmay include both detecting means before and behind the person beingscanned for simultaneously providing signals representative of bothradiant energy transmission and scattering. Appropriately combining suchsignals may help increase the ability of the system to detect a widevariety of concealed obects.

With reference now to FIG. 2, there is shown a view through section 2-2of FIG. 1 illustrating the preferred cross section of the slit in slitcollimator 24. The entrance side of the slit is bounded by beveled edges31 and 32 and the exit side, by beveled edges 33 and 34. Edges 31 and 32are parallel to edges 34 and 33, respectively, so that as scanningoccurs over sector 13, the thickness of the fan beam emitted on the exitside remains substantially constant. The cross section of the slit maytake alternate forms dependent upon the angle being scanned, but ispreferably arranged so that the separation between planes co-extensivewith a pair of parallel faces is substantially the same. Alternately, ifit were desired to angularly scan from a horizontal position, only onepair of edges would be beveled.

Referring to FIG. 4, there is shown a combined pictorial schematicrepresentation of a preferred form of detector 21. The detectorcomprises a number of cascaded segments of sodium iodide or cesiumiodide crystals, such as 35-37, imbedded in light pipe material,

. assembly is encased in X-ray opaque material, such as lead, with onlythe faces of segments 35-37 exposed through an X-ray transparent slit.

This form of detector has a number of advantages. It is nearly percentefficient in terms of picking up all the visible light radiated by theimbedded sodium iodide crystal elements. It is practical and relativelyinexpensive to provide the relatively small crystals that are imbeddedin the lengthwise slit of light pipe 38.

A specific embodiment of the invention may provide a dose delivered to asubject per image of only 0.01 mR, provide a spatial resolution of 3mm3mm, scan an image plane 3 X 6 feet, provide an image in 4 seconds on atelevision monitor, provide an imaging X-ray detection efficiency ofgreater than 99 percent and background detection efficiency of less than1 percent. An important feature of the invention is that scatteredradiation is effectively reduced by 99 percent or more without the useof any special equipment by having a very narrow lead-formed entranceslit along the length of the detector without interfering with theimaging process in any way. The width of this slit need only be the sizeof the pencil beam, typically not more than one five-hundredth of theimage itself. At any instant of time only about one five-hundredth ofthe total scattered radiation is detected, and only this muchcontributes to the background level. Since the scattered intensity isnever more than 10 or 20 times the primary intensity for even thethickest regions of the body, the techniques according to the inventioneliminate at least 99 percent of the scatter to produce primary imageswhich are virtually scatter free to improve the contrast obtained whenthick regions of the body, such as the abdomen and pelvic area areexamined. This feature helps allow metallic objects concealed over theseregions to be detected. And scatter reduction is effective over a widerange of X-ray energies.

Some typical parameter values for an exemplary system aresource-detector distance of 7 feet, detector width 3 feet, separation ofmodulation disc 25 from source 22, 18 inches, imaging plane 6 feet highx 3 feet wide, modulating disc 25 diameter 18 inches with six slitsrotating at 1,800rpm to generate 720 horizontal scan lines in a 4 secondvertical scan with the slits ofthe order of 0.4mm wide and the focalspot of the X-ray tube 22 substantially 0.5mm. The resulting 3mm spotsize facilitates detecting a knife on edge only a few milliemters widein one dimension. Preferably photodetectors 41 and 42 arephotomultipliers providing outputs that may be appropriately amplifiedto provide an ordinary video signal for display on an image storagetube. X-ray tube 22 may be a conventional high-power rotating anode tubetypically used universally for medical diagnostic radiology and operatedtypically at 150kv amd 80ma.

There has been described a novel radiant energy imaging systemcharacterized by. exceptionally low radiation, high resolution, ease ofoperation and assembly and other features. It is evident that thoseskilled in the art may now make numerous uses and modifications of anddepartures from the specific embodiments described herein withoutdeparting from the invention concepts. Consequently, the invention is tobe construed as embracingeach and every novel feature and novelcombination of features present in or possessed by the apparatus andtechniques herein disclosed and limited solely by the spirit and scopeof the appended claims.

What is claimed is:

1. Radiant energy imaging apparatus comprising,

a source of radiant energy,

radiant energy detecting means for providing a signal representative ofradiant energy upon a curve defined thereby,

means for supporting said source and said detecting means in fixedrelative relationship for pivotal movement about an axis included in anaperture plane therebetween,

means defining an aperture in said aperture plane for movement along apredetermined path embracing said axis for scanning said curve with abeam of radiant energy from said source,

and means for angularly displacing said means for supporting about saidaxis over a predetermined angle for coacting with the aperture definingmeans for two-dimensionally scanning an object between said apertureplane and said detecting means to produce a sequence of image signalsrepresentative of the radiant energy response of the medium in a regiontraversed by said beam of radiant energy along a path from said sourceto said radiant energy detecting means.

2. Radiant energy imaging apparatus in accordance with claim 1 whereinsaid means defining an aperture is formed with at least one pair ofbeveled edges included in parallel planes separated by a distancecorresponding to the thickness of a radiant energy beam emitted throughsaid aperture.

3. Radiant energy imaging apparatus in accordance with claim 2 whereinthere are two pairs of said beveled edges with one edge in each pairbeing at the input of said aperture and the other edge of each pairbeing at the output of said aperture.

' 4. Radiant energy imaging apparatus in accordance with claim 1 whereinsaid means for supporting comprises an arm pivotable about said axiscarrying said detecting means at one end and said source at the otherend and said means defining an aperture comprises,

means defining a stationary slit for transmitting radiant energytherethrough and a rotatable disc formed with radial slits thereinadjacent to said stationary slit so that rotation of said disc causessaid radial slits to sequentially scan from one end of said stationaryslit to the other for scanning said curve.

5. Radiant energy imaging apparatus in accordance with claim 4 andfurther comprising a television system responsive to the signal providedby said detecting means for displaying an image representative of theresponse of an object between said detecting means and said apertureplane to said radiant energy.

6. Radiant energy imaging apparatus in accordance with claim 1 whereinsaid radiant energy detecting means comprises,

a light pipe with radiant energy-to-light transducing crystals imbeddedtherein,

and photoelectric transducing means responsive to the light emitted bysaid transducing crystals for providing a video signal.

7. Detecting means in accordance with claim 6 wherein said photoelectrictransducing means is located at at least one end of said light pipe.

8. Detecting means in accordance with claim 7 wherein said photoelectrictransducing means is located at both ends of said light pipe.

9. Detecting means in accordance with claim 6 wherein said light pipe iscircularly cylindrical and formed with a longitudinal slit along thelength thereof in which said crystals are imbedded.

10. Detecting means in accordance with claim 9 wherein said light pipeis formed with opposed end faces and said photoelectric transducingmeans comprises photocells adjacent each end face.

1. Radiant energy imaging apparatus comprising, a source of radiantenergy, radiant energy detecting means for providing a signalrepresentative of radiant energy upon a curve defined thereby, means forsupporting said source and said detecting means in fixed relativerelationship for pivotal movement about an axis included in an apertureplane thErebetween, means defining an aperture in said aperture planefor movement along a predetermined path embracing said axis for scanningsaid curve with a beam of radiant energy from said source, and means forangularly displacing said means for supporting about said axis over apredetermined angle for coacting with the aperture defining means fortwo-dimensionally scanning an object between said aperture plane andsaid detecting means to produce a sequence of image signalsrepresentative of the radiant energy response of the medium in a regiontraversed by said beam of radiant energy along a path from said sourceto said radiant energy detecting means.
 2. Radiant energy imagingapparatus in accordance with claim 1 wherein said means defining anaperture is formed with at least one pair of beveled edges included inparallel planes separated by a distance corresponding to the thicknessof a radiant energy beam emitted through said aperture.
 3. Radiantenergy imaging apparatus in accordance with claim 2 wherein there aretwo pairs of said beveled edges with one edge in each pair being at theinput of said aperture and the other edge of each pair being at theoutput of said aperture.
 4. Radiant energy imaging apparatus inaccordance with claim 1 wherein said means for supporting comprises anarm pivotable about said axis carrying said detecting means at one endand said source at the other end and said means defining an aperturecomprises, means defining a stationary slit for transmitting radiantenergy therethrough and a rotatable disc formed with radial slitstherein adjacent to said stationary slit so that rotation of said disccauses said radial slits to sequentially scan from one end of saidstationary slit to the other for scanning said curve.
 5. Radiant energyimaging apparatus in accordance with claim 4 and further comprising atelevision system responsive to the signal provided by said detectingmeans for displaying an image representative of the response of anobject between said detecting means and said aperture plane to saidradiant energy.
 6. Radiant energy imaging apparatus in accordance withclaim 1 wherein said radiant energy detecting means comprises, a lightpipe with radiant energy-to-light transducing crystals imbedded therein,and photoelectric transducing means responsive to the light emitted bysaid transducing crystals for providing a video signal.
 7. Detectingmeans in accordance with claim 6 wherein said photoelectric transducingmeans is located at at least one end of said light pipe.
 8. Detectingmeans in accordance with claim 7 wherein said photoelectric transducingmeans is located at both ends of said light pipe.
 9. Detecting means inaccordance with claim 6 wherein said light pipe is circularlycylindrical and formed with a longitudinal slit along the length thereofin which said crystals are imbedded.
 10. Detecting means in accordancewith claim 9 wherein said light pipe is formed with opposed end facesand said photoelectric transducing means comprises photocells adjacenteach end face.